The chemical synthesis of (5'C13) adenosylcobalamin has been completed. The chemical synthesis and FT NMR of several cobalamins labeled with C13 in the organs ligands has been undertaken. Adenosyl- and methyledicobalamin have been synthetized and tested in the ribonucleotide reductase and methyltransferase systems. A large series of analogs of adenosylcobalamin have been synthesized (Biochemistry 13, 2736 (1974)) and these analogs have been tested for coenzymes and for inhibitor activity on the ribonucleotide reductase system. Studies are presently in progress which involve the PRFT NMR of the C13 labeled corrinoids. FT NMR has also been used in the thermal and photolytic decomposition of the carbon-cobalt bond.